Hard or abrasive workpiece materials, such as metal alloys, ceramics, cermets, certain composite materials and stone may need to be machined using tools having hard or superhard cutting tips. Cemented tungsten carbide is the most widely used tool material for machining hard workpiece materials, and is both hard and tough. Polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) are superhard materials, which may be used for machining certain metal alloys widely used in the automotive industry, for example. Superhard materials are extremely hard and may have Vickers hardness of at least about 25 GPa. However, superhard materials are typically less strong and tough than cemented carbide materials and consequently, they may be more prone to fracture and chipping than hard-metals. Superhard cutter inserts may comprise a superhard structure bonded to a support substrate, which is most typically formed of cemented tungsten carbide.
U.S. Pat. No. 3,745,623 discloses the manufacture of PCD in a titanium or zirconium protective sheath, some of which is converted to carbide during manufacture. A thin layer of this titanium or zirconium sheath may be left on the PCD over the chip breaker face.
U.S. Pat. No. 5,833,021 discloses a polycrystalline diamond cutter, particularly for use in drill bits for boring into the earth, having a refractory coating applied to the polycrystalline diamond surface to increase the operational life of the cutter.
U.S. Pat. No. 6,439,327 discloses a polycrystalline diamond cutter for a rotary drill bit for boring into the earth, in which a side surface of the cutter is provided with a metal layer high-pressure bonded to the side surface of the polycrystalline diamond. An example of a suitable metal is molybdenum.
U.S. Pat. No. 6,779,951 discloses a drill insert component made by sandwiching polycrystalline diamond (PCD) between a layer of cemented tungsten carbide, or similar hard material, and a multi-metal layer for improved interlayer bonding and a lower propensity for delamination.
PCT patent publication number WO/2008/104946 discloses a tool component comprising a layer of polycrystalline diamond having a working surface, and a softer layer having a thickness of up to 100 microns and containing a metal.
PCT patent publication number WO/2008/104944 discloses a method of machining a workpiece using a tool that includes a tool component comprising a layer of polycrystalline diamond having a working surface, and a softer layer containing a metal and bonded to the working surface.
United Kingdom patent publication number 2 251 879A discloses a method of making a preform cutting element for rotary drill bits for drilling or coring holes in subsurface formations. The method comprises the steps of forming an intermediate structure comprising superhard material bonded between two outer layers of less hard material. The superhard material preferably comprises polycrystalline diamond material, and the less hard material preferably comprises tungsten carbide.
There is a need to provide a superhard tool for cutting hard or abrasive workpieces to a tight tolerance finish, having a very smooth surface finish to a high level of dimensional accuracy, with reduced damage to the edges of the workpiece, and consequently enhanced tool life.